Memory devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied.
A typical example of a volatile memory device is a static random access memory (SRAM). Since an SRAM holds stored data with a circuit such as a flip flop, the number of elements per memory element is increased (for example, six transistors per memory element); therefore, cost per storage capacity is increased.
Another example of a volatile memory device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a memory cell is selected and charge is accumulated in a capacitor. In general, a DRAM is used as an element which stores one bit (two values) of data. However, a DRAM can be used as an element which stores two or more bits (four or more values) of data when there are four or more levels of the amount of charge accumulated in a capacitor of the DRAM (e.g., see Patent Document 1).